This invention relates to a novel process for preparing certain aminoplast resins, more particularly, advanced etherified aminoplast resins containing one or more guanamine residues, to the resins made by the new process, and to their use in coating compositions.
Etherified aminoplast resins are, in general, well known, commercially-available materials and are used in numerous fields of application, especially in liquid or powder coating compositions. They are prepared by reacting an amido compound with an aqueous solution of formaldehyde or with a formaldehyde donor, e.g., paraform, under neutral or basic conditions to form the methylolated amido compound, i.e., the aminoplast, and etherifying this with an alkanol under acid conditions. Such materials are often blended with an alcoholic hydroxyl group-containing resin, such as an alkyd, polyester, or acrylic resin, and cured at elevated temperatures in the presence of an acid catalyst, or, when larger proportions of an acid catalyst are used, at room temperature. Epoxide resins of higher molecular weight which contain alcoholic hydroxyl groups as well as epoxy groups are also used. Other resins employed as the coreactant are carboxyl-containing or carboxylic acid amide-containing resins.
For certain applications, such as in the formulation of powder coatings, it is highly desirable that the etherified aminoplast resin be a non-sticky solid at room temperature or at slightly elevated temperatures. Many commercially-available etherified aminoplast resins are sticky, highly viscous liquids at ordinary temperatures, and so cannot be used in powder coatings.
Further, it is known that the compatibility of etherified aminoplast resins with a range of coreactants and solvents is dependent, to a large extent, on the nature of the etherifying group. When the etherifying group is derived from a lower alkanol, particularly methanol, good compatibility may not always be achieved.
We have now found a novel method of making etherified aminoplast resins which are solid at ambient and moderately elevated temperatures and which are fully compatible with a wide range of coreactants and solvents.
It is known, from British Patent Specification No. 948,853, to prepare a modified aminoplast resinous composition by
i. forming a partially polymerised aminotriazine resin comprising an aldehyde, especially formaldehyde, and an aminotriazine, which is usually melamine but may be a guanamine,
ii. adding a small amount of an aminotriazine to the partially polymerised aminoplast resin (which is unetherified),
iii. heating the resulting syrup for a short period, and
iv. cooling the hot syrup, and then blending therewith a small amount of one or more amines.
The product is used alone to form laminates from glasscloth.
British Patent Specification No. 1,048,710 describes the reaction of benzoguanamine and formaldehyde to form a monomeric reaction product which is then treated with melamine and a polymerisation catalyst is added. The product is likewise unetherified.
British Patent Specification No. 1,141,972 describes the reaction of a methylolated melamine with acetoguanamine and a toluenesulphonamide to give a product suitable for mouldings and laminates, having good reforming properties and cracking resistance. In the aforesaid specification, for purposes of comparison, a product is described which was prepared from methylolated melamine and acetoguanamine only, and this product was shown to be unsuitable as a laminating resin as it was unmouldable or gave a poor surface glaze. This product, like the others mentioned in the Specification, was unetherified.
In the process now provided, an etherified, methylolated urea, cyclic urea, carbamate, or melamine is heated with, as advancing agent, a monomeric guanamine, under mildly acid to mildly basic conditions. The term "advancement" in its various forms is used herein in its conventional sense to refer to a process in which a substantially uncrosslinked product of higher molecular weight is produced.